We have a brand, let's call it "ourbrand". Domain name registered 4 years ago and we think that the brand is getting known very well, day after day.
2 years ago we launched a subdomain: [region]business.ourbrand.com. Started to advertise this new subdomain and it became very popular, actually holds the top positions in google for many competitive "region + ourwords" terms. We have many sites linking to this subdomain and internal pages. The problem is that the subdomain [region]business.ourbrand.com is a little long and probably hard to remember. It would be also hard to build a regional brand around a subdomain name.
So, what we did? We registered a new domain name [region]ourbrand.com. Now, this domain name is a simple presentation page where we actually link to [region]business.ourbrand.com. We would like to get rid of the subdomain and build a brand around the domain name, we would like our members and clients to use [region]ourbrand.com domain name instead the subdomain name.

Some facts:
- the subdomain has thousands of pages indexed in google and as stated above, we are in top of google with many competitive terms
- yahoo does not list our subdomain, so we are nowhere at yahoo, msn does list us in top positions for our main terms (3-5)
- we have many incoming links to the subdomain.

What would be the safest way to get rid of subdomain and use the domain name instead? I mean, a 301 redirect from the subdomain page to the domain name would be an easy task, but how will this be treated by google?
Is the PR passed to the new domain name? Will our google rankings stand where they are?